A vibration sensor in which a thin film is formed by stacking layers of monocrystalline silicon or polycrystalline silicon on a silicon substrate, and the thin film is used as a diaphragm, is being developed by applying a semiconductor integrated circuit manufacturing technique. The diaphragm made of silicon has a small internal stress and a low density compared to metal such as aluminum and titanium, and thus a vibration sensor of high sensitivity can be obtained and compliance with a semiconductor integrated circuit manufacturing step is also satisfactory. The vibration sensor including such a diaphragm includes a capacitor microphone disclosed in Patent Document 1. In such a capacitor microphone, a diaphragm (movable electrode) and a fixed electrode are formed on a front surface of a semiconductor substrate of a monocrystalline silicon (100) plane, and thereafter, an etching mask is formed at the outer peripheral part of a back surface of the relevant semiconductor substrate, and the semiconductor substrate is etched from the back side until reaching the front surface thereby forming a through-hole at a central part of the semiconductor substrate. As a result, the diaphragm has a periphery fixed to the front surface of the semiconductor substrate and the central part supported in a hollow state above the through-hole, and thus can be vibrated by audio vibration and the like.
However, in the capacitor microphone of such a structure, an inclined surface from a (111) plane appears at the inner peripheral surface of the through-hole since the (100) plane semiconductor substrate is subjected to crystal anisotropic etching from the back side, and the through-hole becomes a space of a truncated pyramid shape opened large to the back side. Thus, the opening area at the back side of the through-hole becomes large compared to the area of the diaphragm, and miniaturization of the capacitor microphone is difficult. If the thickness of the semiconductor substrate is reduced, the ratio of the opening area of the back surface with respect to the opening area of the front surface of the through-hole can be reduced, but the strength of the semiconductor substrate lowers and handling at the time of manufacturing becomes difficult, and thus reducing the thickness of the semiconductor substrate has limits.
In a film-type sensor described in Patent Document 2, a through-hole is opened in the semiconductor substrate from the back side by vertical etching such as a so-called DRIE (Deep Reactive Ion Etching) and ICP (Inductively Coupled Plasma). Therefore, the film-type sensor can be miniaturized by an amount the through-hole does not spread in the truncated pyramid shape. However, in devices of DRIE, ICP, and the like, a price of the device is high and processing of a wafer is in sheet form, and thus productivity is not satisfactory.
A method for performing crystal anisotropic etching on the semiconductor substrate from the front side (diaphragm side) is also known, but in such a method, an etching hole needs to be opened in the diaphragm, which etching hole sometimes adversely affects vibration characteristics, strength, and the like of the diaphragm.
An etching method for forming a through-hole in the substrate by performing crystal anisotropic etching on the (100) plane semiconductor substrate from the back side, and reducing the ratio of the opening area of the back surface with respect to the opening area of the front surface includes a method described in Patent Document 3. In such a method, a sacrifice layer is first formed in a rectangular region to be opened on the front surface of the silicon wafer or the semiconductor substrate, and a thin film of silicon nitride is vapor deposited thereon, and thus the opening area of the back surface of the silicon wafer merely needs to be a size the through-hole can reach the sacrifice layer by anisotropic etching.
However, in this etching method, the thin film is exposed to an etchant of the substrate when etching the substrate of after the sacrifice layer of the front surface is etched, and thus monocrystalline silicon and polycrystalline silicon cannot be used for the material of the thin film. Furthermore, since the thin film is directly formed on the substrate, steps and structures for enhancing the characteristics of the vibration sensor such as stress control of the thin film and vent hole become difficult to incorporate. Therefore, the etching method disclosed in Patent Document 3 is not suitable for the method for manufacturing the vibration sensor requiring high sensitivity such as microphone.
Patent Document 1: Japanese Unexamined Patent Publication No. 2004-506394
Patent Document 2: Japanese Unexamined Patent Publication No. 2003-530717
Patent Document 3: Japanese Unexamined Patent Publication No. 1-309384